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Networking
• Basic network components and layered
architecture
• Internet and WWW basics
• IP, TCP, URL, HTTP
Jean Walrand, Communication Networks, a first course, McGraw-Hill 1998 (2nd edition).
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Types of computer networks:
• Point-to-point connection (link) between two computers
e.g. PPP, SLIP
• Store-and-forward transmission: packets
A
B
C
e.g. token ring
• Datagram packet switching: destination address in packet
• also, LAN, WAN, ATM (virtual circuit switching), wireless
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• multiple access networks (e.g. ALOHA, Ethernet)
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Review of components
•
Hosts
–
–
•
Hubs
–
–
•
depending on level of switch complexity, anywhere from a hub to a router
Routers
–
–
•
attaches two physically identical LANs together, physical layer processing
forwards only traffic which is destined for “the other side”
Switches
–
•
acts as an “extender” - similar in effect to taking all incoming lines and twisting the
wires together
Does not do any processing - acts only at the physical layer
Bridges
–
–
•
Considered a “node” or “end point” in the network (client/server/printer)
Processes all levels of the protocol stack
Processing at the Network layer
Route packets between networks with potentially different lower level protocol
stacks (i.e., different physical and data link layers)
Gateways
–
–
Processes all levels of the protocol stack
Used to connect networks with different protocol stacks
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Layered communication model
Message received
Message sent
Layer n
Layer 2
Layer 1
Sender
Communication
medium
Recipient
Coulouris, Dollimore, Kindberg, Distributed Systems, Addison Wesley 2001 (3rd ed)
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ISO-OSI layer model
Open systems interconnection reference model
A
B
communication services
7.
Application
7.
local syntax, secure,
efficient connections
6.
Presentation
6.
communication services, e.g. file transfer,
email, …
e.g. compression, security, format conversion
connections
5.
Session
5.
supervises connections between end systems
delivery of messages
4.
Transport
4.
supervises end-to-end transmission
3.
Network
3.
guides the packet from source to destinations
2.
Data Link
2.
implements packet delivery service between two
nodes on the same physical link
1.
Physical
1.
Implements digital communication link that
delivers bits
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Layer
Application
Presentation
Session
Transport
Network
Data link
Physical
Description
Protocols that are designed to meet the communication requirements of
specific applications, often defining the interface to a service.
Protocols at this level transmit data in a network representation that is
independent of the representations used in individual computers, which may
differ. Encryption is also performed in this layer, if required.
At this level reliability and adaptation are performed, such as detection of
failures and automatic recovery.
This is the lowest level at which messages (rather than packets) are handled.
Messages are addressed to communication ports attached to processes,
Protocols in this layer may be connection-oriented or connectionless.
Transfers data packets between computers in a specific network. In a WAN
or an internetwork this involves the generation of a route passing through
routers. In a single LAN no routing is required.
Responsible for transmission of packets between nodes that are directly
connected by a physical link. In a WAN transmission is between pairs of
routers or between routers and hosts. In a LAN it is between any pair of hosts.
The circuits and hardware that drive the network. It transmits sequences of
binary data by analogue signalling, using amplitude or frequency modulation
of electrical signals (on cable circuits), light signals (on fibre optic circuits)
or other electromagnetic signals (on radio and microwave circuits).
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Examples
HTTP, FTP , SMTP,
CORBA IIOP
Secure Sockets
(SSL),CORBA Data
Rep.
TCP, UDP
IP, ATM virtual
circuits
Ethernet MAC,
ATM cell transfer,
PPP
Ethernet base- band
signalling, ISDN
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What is a protocol
`A protocol is a set of rules governing message
interchanges which occur between a number of
computers in a distributed system; each of these
messages implement functions of the system. A
protocol is also used to establish a connection and
allow entities to exchange data about themselves, for
example the operating system they are using and the
format of the data that they intend passing; this is
known as a handshake. ‘
D. Ince
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Application message
TCP header
port
IP header TCP
Ethernet header IP
Ethernet frame
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IP Header
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• Around 4 billion IP addresses
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Transport layer protocols
• TCP (Transmission Control Protocol):
– connection-oriented
– Reliable packet delivery in sequence
• UDP (User Datagram Protocol):
–
–
–
–
–
–
connectionless (datagram)
Unreliable packet delivery
Packets may arrive out of sequence or duplicated
Less overhead
Simply adds port addressing to IP
Checksum is optional
• Internet Control Message Protocol
– Used by the nodes in the internet to implement IP
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TCP and UDP Header
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Name
Port/Protocol
ftp-data
20/tcp
ftp
21/tcp
telnet
23/tcp
smtp
25/tcp
bootps
67/udp
bootpc
68/udp
# BOOTP/DHCP client
hostnames
101/tcp
hostname
sunrpc
111/udp
rpcbind
sunrpc
111/tcp
rpcbind
finger
79/tcp
pop3
110/tcp
# Post Office
nntp
119/tcp
usenet
imap
143/tcp
# Internet Message Access
ssh
22/tcp
# Secure SHell
who
513/udp
whod
kerberos
750/udp
kdc
xaudio
1103/tcp
Xaserver
stel
10005/tcp
# Secure Telnet
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Comment
mail
# BOOTP/DHCP server
# usually to sri-nic
# Network News Transfer
# Kerberos key server
# X Audio Server
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The Internet around 1990
Jean Walrand, Communication Networks, a first course, McGraw-Hill 1998 (2nd edition).
The first three chapters give you a nice overview of the working of networks and the Internet.
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MCI backbone
Retrieved from: http://global.mci.com/about/network/maps/?flash=1&theme=/, Sept 5/2003
See this site also for interesting network statistics (e.g. latencies)
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Internet network architecture
ISP
ISP
ISP
ISP
NAP
NAP
ISP
ISP
NAP
NAP
ISP
Backbone
Network Service Provider
ISP
ISP
ISP
Internet Service Provider
Private Networks / hosts
Adapted from Electronic Commerce: A Managerial Perspective. Turban, Lee, King and Chung, 2000. Pg. 385
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How big is the Internet today?
– 171,638,297 connected computers (Jan 2003)
“Source: Internet Software Consortium (http://www.isc.org/)”
– 605.60 million people online Worldwide (Sep 2002)
•
•
•
•
•
•
Canada & USA 182.67 million
Europe 190.91 million
Asia/Pacific 187.24 million
Latin America 33.35 million
Africa 6.31 million
Middle East 5.12 million
Source: Nua Internet Surveys; retrieved from:
http://www.nua.ie/surveys/how_many_online/ Sept. 5/2003
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a.cs.dal.ca
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b.atr.go.jp
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URL: A Global Address
http://www.cs.dal.ca/cs1200/week1/x.html
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•
•
•
•
Scheme
Server name
Path
File
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a.cs.dal.ca
123.777.19.6
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b.atr.go.jp
168.12.13.3
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Routing and congestion control
Send information of nodes and links to each node of the network. This might
include some physical properties such as maximal size of packets, typical time
delays, …
 Build map of network
 Use routing algorithm to build routing tables
for example: OSPF (Open Shortest Path First)
if every router uses the same algorithm and maps they build consistent tables
 Hierarchical Routing:
`use local map to get to the highway, use highway map to get to your
destination town, use local map to get to your friends house’.
More sophisticated routing on `highway’ (BGP, Border Gateway Protocol)
How is congestion controlled?
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How is congestion controlled?
Flow control  TCP
Destination publishes maximal acceptable window size in reply
message
Source destination discovers congestion from unusually long delay
times of response
 additive increase, multiplicative decrease
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Application layer:Programming example in Java
The JAVA Socket class
Socket oldSock = new Socket("penny.open.ac.uk", 1048);
remote computer penny in the domain open.ac.uk with communication occurring
via port 1048.
 InputStream
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Packet filtering firewalls
–Packet filtering firewalls decide whether or not to forward packets based on their
source and destination IP addresses and port numbers
–Rules dictate whether or not packets should be forwarded
–Typically once a connection through the firewall has been established, further
packets are passed without scrutiny
–Processes up to the network layer of the protocol stack (one notable exception is for
FTP, which requires some application-level support)
perform
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IP Masquerading
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Proxy-based firewalls
– Proxy-based firewalls operate at the application layer of the protocol stack
– Every type of application for which a connection through the firewall is
requested requires that a proxy server be running on the firewall for that
specific application, or the request will be denied
– Allows for logging of events at the application layer, much more detailed
logging than a packet-filtering firewall allows
– Requires that client machines inside the firewall be configured on an
application by application basis to use the proxied services of the firewall
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